The invention relates to the general field of aviation.
More particularly, it relates to estimating parameters in an aircraft turbojet, such as the temperature of a fluid, for example.
A preferred but non-limiting application of the invention lies in the field of systems for regulating and controlling turbojets.
In known manner, in order to regulate and adapt the control of a turbojet to various flight constraints, it is necessary to estimate the temperature of various gas streams passing through the turbojet (referred to as stream temperatures). For this purpose, use is made of temperature sensors such as probes or thermocouples that are positioned in various locations in the gas streams and that are adapted to measure the temperatures of said gas streams.
Temperature sensors generally suffer during a measurement from thermal inertia that is specific to each sensor and that depends in particular on the mass or the size of the sensor. This inertia is reflected in a time shift between the moment at which the measurement is effected by the sensor and the moment at which it delivers a signal in response to that measurement. This is referred to as the measurement lag effect and can cause malfunctions of the jet engine because of measurement mismatch, in particular during rapid variations in the temperatures of the gas streams.
In order to avoid such a malfunction, it is possible to envisage using sensors that present very low inertia. Nevertheless, such sensors are very expensive.
To alleviate this problem there exist techniques for correcting the measurement signals delivered by a temperature sensor that compensate the lag effect induced by the inertia of the sensor. One such technique is described in U.S. Pat. No. 5,080,496, for example.
Those techniques generally rely on digital modeling of the inertia of the sensor using a filter with parameters set by estimating the time constant of the sensor. As is known in itself, the time constant of a measurement sensor characterizes its response time, i.e. its inertia.
Prior art techniques for estimating the time constant of a temperature sensor use fixed graphs depending on one or more parameters, for example the flow rate of the fluid in which the sensor is placed. Those graphs indicate mean values of time constants for response time templates and predetermined conditions. In other words, they do not in fact take account of the spread of inertia from one temperature sensor to another.
Current fabrication technologies do not enable temperature sensors for controlling jet engines to be produced at low cost and that also comply with a response time template subject to little spread. Consequently, it is difficult to obtain graphs adapted to the various temperature sensors concerned. Numerous problems have arisen when the time constants of the sensors mounted in a jet engine depart considerably from the values given by these graphs.
One solution would be to test each temperature sensor, for example in a wind tunnel, to determine its time constant under predefined conditions, and to extrapolate the graphs as a function of the time constants determined in this way. Such a test is particularly costly, however, and represents approximately one-third of the price of the temperature sensor. Consequently, it cannot be used for each temperature sensor, which means that a temperature sensor outside an acceptance template for which a graph is available might not be detected.
Furthermore, such tests are often carried out at fluid flow rates limited by the capacities of a wind tunnel and they are generally not able to cover the range of working flow rates in jet engine applications. Extrapolating graphs to cover all the range of working flow rates introduces inaccuracies into the acquisition system of the temperature sensor.
Moreover, as mentioned above, the time constant of a temperature sensor depends on parameters such as the flow rate of the fluid in which the sensor is placed. This means that in order to estimate the time constant of a temperature sensor it is necessary first to estimate this fluid flow rate. Consequently, it is necessary to use additional estimator modules on the jet engine, which makes correcting measurements even more complex.
Consequently, there exists a need for a method of estimating a stream temperature in a turbojet, which method is simple and inexpensive and delivers an estimate of said temperature that is accurate so as to be suitable for use in particular in regulating and controlling the turbojet.